This invention relates to the upgrading of the olefinic by-product stream produced in low-severity catalytic hydrocarbon conversion processes. In particular, this invention relates to upgrading the olefinic by-product stream produced by the catalytic dewaxing of waxy hydrocarbon feedstocks in the presence of a medium-pore zeolite catalyst.
Catalytic dewaxing of hydrocarbon oils to reduce the temperature at which precipitation of waxy hydrocarbons occurs is a known process and is described, for example, in the Oil and Gas Journal, Jan. 6, 1975, pages 69-73. A number of patents have also described catalytic dewaxing processes. U.S. Pat. No. Reissue 28,398 describes a process for catalytic dewaxing with a catalyst comprising a medium-pore zeolite and a hydrogenation/dehydrogenation component. U.S. Pat. No. 3,956,102 teaches a process for hydrodewaxing a gas oil with a medium-pore zeolite catalyst. U.S. Pat. No. 4,100,056 discloses a Mordenite catalyst containing a Group VI or a Group VIII metal which may be used to dewax a distillate derived from a waxy crude. U.S. Pat. No. 3,755,138 describes a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point. Such developments in catalytic dewaxing have led to the MLDW (Mobil Lube Dewaxing) and MDDW (Mobil Distillate Dewaxing) processes. The entire contents of the above-listed publications and patents are incorporated by reference as if set forth at length herein.
Dewaxing is typically a two-step process comprising catalytic dewaxing followed by hydrotreating. Certain feedstocks, particularly distillate feedstocks, however, may meet product specifications without hydrotreating. In lubricant manufacturing, hydrotreating improves color and reduces sulfur content of the finished product. Hydrotreating also saturates the olefinic by-products from the dewaxing reaction and is typically used for this purpose as well as to reduce sulfur and to increase cetane number in distillate products. For example, U.S. Pat. No. 3,668,113 describes a catalytic dewaxing process employing a Mordenite dewaxing catalyst which is followed by a catalytic hydrodesulfurization step over an alumina-based catalyst. U.S. Pat. No. 4,400,265 describes a catalytic dewaxing/hydrodewaxing process using a zeolite catalyst having the structure of ZSM-5 wherein gas oil is catalytically dewaxed followed by hydrodesulfurization in a cascade system. Hydrotreating processes are widely used in the petroleum refining industry and are exemplified by the processes described in U.S. Pat. Nos. 4,054,508 to Milstein et al.; 4,267,071 to Jaffe; and 4,600,503 to Angevine et al., each of which is incorporated by reference as if set forth at length herein.
Commonly-assigned application Ser. No. 125,435, filed Nov. 11, 1987, teaches a hydrodewaxing process with interstage recovery of olefinic gasoline and is incorporated by reference as if set forth at length herein. The hydrodewaxing process described comprises a first catalytic dewaxing reaction zone containing a medium-pore zeolite catalyst and a second hydrotreating reaction zone. By separating the olefinic naphtha prior to the hydrotreating step, hydrogen consumption is reduced and a smaller hydrotreating reactor may be used.
By-products of these catalytic dewaxing processes include highly olefinic gasoline having a research clear octane number in the range of 80 to 95. Motor octane numbers for this olefinic gasoline stream typically range from about 65 to 80. Road octane numbers for finished gasoline product are calculated as the arithmetic mean of the research and motor octane numbers. At the present time, road octane numbers for gasoline sold at retail equal or exceed 87 and generally range between 87 and 93. Highly olefinic gasoline is therefore unsuitable for use as a gasoline blending component. Further, the olefin contents of more than 20 wt. % preclude economic upgrading of this stream via catalytic reforming.
Catalytic reforming is widely used to increase octane in gasoline boiling range feedstocks. Paraffinic feedstocks are more easily upgraded in a catalytic reformer than olefinic feedstocks. Olefinic feedstocks tend to form excessive amounts of coke in the reformer reactors and cause more rapid deactivation of the reforming catalyst. Consequently, reformers are typically equipped with pretreaters which catalytically react naphtha feedstock with hydrogen to saturate olefins and to remove sulfur compounds which poison the reforming catalyst. Hydrogen consumption is related to the concentration of olefinic compounds in pretreater feed and, as a result, olefinic feeds consume significantly more hydrogen during pretreatment than paraffinic feeds, making olefinic feeds more costly to pretreat.
The economic benefit of the catalytic reforming octane boost is offset by liquid product yield loss. This loss becomes more pronounced as reaction severity increases until the economic detriment of yield loss overshadows the economic advantage of the octane boost. For a general discussion of naphtha reforming, see 17 Kirk Othmer Encyclopedia of Chemical Technology, 218-220, 3rd edition, 1982.
Thus, the gasoline stream produced in a catalytic dewaxing unit is relatively difficult and expensive to upgrade in a catalytic reforming process. Further, the catalytic reforming process units are typically sized to accommodate the available paraffinic naphtha feed, and lack capacity to process a supplemental olefinic feedstock.
Olefinic gasoline streams may be readily upgraded to high octane gasoline via catalytic aromatization as disclosed in U.S. Pat. No. 3,756,942 to Cattanach and U.S. Pat. No. 3,759,821 to Brennan et al., the disclosures of which are incorporated by reference as if set forth at length herein.
The present process overcomes the disadvantages of previous processes by aromatizing the low-octane highly olefinic gasoline by-product of a low-severity catalytic dewaxing process to form a high-octane gasoline product. Advantageously, the olefinic gasoline by-product is upgraded by charging the gasoline to the catalytic dewaxing reaction zone preferably when the dewaxing catalyst is at least partially deactivated. The present invention enables the refiner to produce a valuable dewaxed distillate or lubricant product as well as a high octane gasoline from a single reactor system.